Delta-sigma (ΔΣ) modulation, sometimes referred to as sigma-delta (ΣΔ) modulation, is a well-known technique for converting (i.e., encoding) analog signals into digital signals, or converting higher-resolution digital signals into lower-resolution digital signals. Delta-sigma modulators (or converters) essentially employ oversampling to reduce the in-band power of quantization noise, and use feedback to shape this noise and move it out-of-band. The intrinsic tolerance of delta-sigma converters to analog circuitry inaccuracy renders them very well-suited for the on-chip design of high-resolution interfaces in mixed-signal application-specific integrated circuits (ASICs).
Delta-sigma modulators of various types have been used for analog-to-digital and digital-to-analog conversion over the last several decades. Moreover, delta-sigma modulators have been used in high-efficiency switching power amplifiers (SWPAs), including, for example, class-D SWPAs, commonly used for digital audio applications, and class-S SWPAs, commonly used for radio-frequency (RF) applications.
It is well-known that higher-order (e.g., an order greater than two) one-bit, single-loop delta-sigma modulators suffer from instabilities. To address this shortcoming, cascaded delta-sigma modulators have been proposed which are constructed by cascading a chain of stable, lower-order (usually one-bit) modulators to build a stable higher-order modulator. The output of a cascaded modulator, fabricated from one-bit modulators, forms a multiple-bit bitstream.
More recently, delta-sigma modulators using Viterbi decoders as quantizers with memory were introduced for digital audio applications. (See, e.g., H. Kato, “Trellis Noise-shaping Converters and 1-bit Digital Audio,” Proceedings of the AES 112th Convention, Munich, preprint 5615, May 10-13, 2002, and P. Harpe, “Trellis-type Sigma-delta Modulators for Super Audio CD Applications,” Research Report, Philips Research, Jan. 29, 2003, the disclosures of which are incorporated herein by reference in their entireties for all purposes.) As regular (i.e., memory-less or non-Viterbi) higher-order one-bit, single-loop delta-sigma modulators are known to suffer from instabilities, higher-order one-bit, single-loop Viterbi bitstream generators are also more prone to instabilities than lower-order one-bit, single-loop Viterbi bitstream generators.